Titanium dioxide (TiO2) is one of the most important metal oxides in industrial applications, since it is used in an array of different sectors, ranging from paper production to pharmaceuticals, cosmetics, photocatalysers, photovoltaic cells, photoelectric cells, sensors, inks, coatings, coverings and plastic, and even encompassing the photocatalysis of organic pollutants. In particular, certain types of TiO2 are especially suitable for applications involving photovoltaic cells, particularly Dye Sensitized Solar Cells (DSSC), photoelectrolysis cells and tandem cells for the conversion of solar energy and the production of hydrogen.
TiO2 has various crystalline shape. The most common crystalline phases of TiO2, anatase, rutile and brookite, exhibit different chemical/physical properties, such as stability field, refraction indexes, chemical reactivities and behaviour to irradiation with electromagnetic radiation. The use and performance of TiO2 depends greatly on its crystalline phase, on its morphology and on the dimensions of the particles, as reported, for instance, by X. Chen and S. S. Mao in J. Nanosci. Nanotechnol, 6(4), 906-925, 2006. The phase composition, the shape of the crystals and the dimensions of the particles exert an enormous influence Over the chemical/physical, mechanical, electronic, magnetic and optical properties of the end product.
In terms of their dimensions, particles with nanometric dimensions have electrical, thermal, magnetic and optical properties that differ from those of larger particles. TiO2 particles with nanometric dimensions, particularly those with a diameter of between 1 and 20 nanometres, have properties similar to those of molecules, in that they exhibit effects of quantisation and unusual luminescence (X. Chen and S. S. Mao, Chem. Rev., 107, 2891-2959, 2007).
Anatase-phase crystalline TiO2 is an oxide that is widely used as a photocatalyser, as a white pigment for coatings and cosmetic products, and in various types of sensors.
The most recent, and most important, uses of anatase TiO2 with nanometric dimensions concern applications involving photovoltaic cells, particularly DSSC, photoelectrolysis cells and tandem cells for the conversion of solar energy and the production of hydrogen.
Based on studies conducted on the specific application of TiO2 in DSSC cells (X. Chen and S. S. Mao, Chem. Rev., 107, 2891-2959, 2007 and J. Nanosci. Nanotechnol, 6(4), 906-925, 2006), it has been demonstrated that the most preferred shape is anatase crystalline nanorods.
Of late, the synthesis of controlled shape nanomaterials based on TiO2 has become the subject of much intense research, and new synthetic methods have been developed that allow the phase composition, morphology and dimensions of the particles to be controlled (X. Chen e S. S. Mao, J. Nanosci. Nanotechnol, 6(4), 906-925, 2006).
The main methods for producing nanorods for industrial use are:
a) hydrothermal synthesis;
b) solvothermal synthesis;
c) sol-gel synthesis.
Hydrothermal syntheses, method a), use aqueous solutions containing titanium tetrachloride, generally in the presence of acids, inorganic salts and surfactants, at temperatures of up to 160° C. (X. Feng et al., Angew. Chem. Int. Ed., 44, 5115-5118, 2005; S. Yang and L. Gao, Chem. Lett. 34, 964-5, 2005; ibid. 34, 972-3, 2005; ibid. 34, 1044-5, 2005). Preferably, it is the rutile phase that is obtained, making these methods unsuitable for the formation of anatase.
Solvothermal synthesis, method b), (C. S. Kim et al., J. Cryst. Growth, 257, 309-15, 2003) makes it possible to obtain nanosized rods with anatase phase composition. These reactions are conducted in autoclave, mostly under anhydrous conditions, at high temperatures of around 250° C., for long periods, using an aromatic solvent, such as toluene, and in the presence of an organic acid such as oleic acid, which also functions as a surfactant. The titanium/solvent/surfactant ratio of the reagents exerts a strong influence over the dimensions of the nanorods, making it a laborious process to reach the desired result. Moreover, the requirement for prolonged thermal treatment makes this method of synthesis an expensive option.
High-temperature reactions using benzyl alcohol as a solvent, and in the absence of acidity (A. P. Caricato et al., Appl. Surf Sci. 253, 6471-6475, 2007), enable the production of particles that are mostly spherical, albeit under rather drastic reaction conditions.
Sol-gel synthesis, method c), involves the controlled hydrolysis of titanium alkoxide with water, in the presence of fatty organic acids, such as oleic acid, which serves as a surfactant and stabilising agent, and catalysts such as amine or quaternary ammonium salts (Cozzoli, P. D., Kornowski, A., Weller, H. J., J. Am. Chem. Soc., 125, 14539-14548, 2003). These reactions occur under relatively mild conditions and afford control over the dimensions of the crystalline-shape particles, but the TiO2 particles obtained are polluted by organic products, rendering them unsuitable for certain applications. The purification of these particles requires, therefore, a prolonged post-treatment calcination process, which, in addition to being costly, could significantly modify the characteristics of the end product, which may not match the requested characteristics.
R. Parra et al., in Chem. Mater, 20, 143-150, 2008, describe the combined use of organic acids with low molecular weight, such as acetic acid, and 2-propanol as a solvent, in the absence of surfactants, to produce anatase-phase TiO2 from titanium tetraisopropoxide. However, this process makes it possible to produce only agglomerates, and not nanorods.
The patent application US 20060104894 describes the production of nanocrystals of anatase TiO2 through the reaction of a titanium precursor and an organic acid, in the presence of an acidic catalyst (e.g. nitric acid) or a basic catalyst, in a solvent including water and alcohols with low molecular weight, heating the resultant solution to 50±15° C. However, no mention is made in this patent application of the shape of the product obtained using this process.
According to patent application US 20060034752, it is possible, through the reaction of a titanium precursor, in the presence of an acid (nitric acid, hydrochloric acid, acetic acid or oxalic acid), in water and alcohols with low molecular weight to produce a hydroxide of titanium that, only after calcination, transforms itself into TiO2, but does so with a mixed-phase anatase/brookite composition.
According to the patent application WO 2007028972, it is possible, through the reaction of an alkoxide of titanium in ethanol or acetone and benzyl alcohol in the presence of water or acetic acid, and only after calcination at 400° C., to produce anatase-phase TiO2, which is subsequently transformed into rutile-phase TiO2 through heating to a temperature between 650° and 950° C.
Water and polyols are used in the patent application WO 2006061367, but this process is not suitable for producing anatase-phase TiO2 nanorods.
In the patent JP 2003267705 on the production of materials coated with a metal oxide, particularly zinc oxide, where the material to be coated is immersed in the reaction mixture, reference is made to the use of acetic acid, benzyl alcohol and titanium n-butoxide as reactants, but no information is provided on the shape of the titanium dioxide obtained, which, in any case, is not isolated as such; rather, it is produced only in the form of a coating for another material.
The optimum solution for the low-cost, industrial-scale production of anatase-phase TiO2 particles with nanometric dimensions and controlled shape, which are highly suitable for applications involving photovoltaic cells, particularly DSSC, photoelectrolysis cells and tandem cells for the conversion of solar energy and the production of hydrogen, has yet to become available. There is, then, a need for a process whereby it is possible to produce nanocrystalline, anatase-phase TiO2 particles with controlled shape.